The term "thick film electrical components" as used hereinafter refers to electrical components such as conductor lines, crossovers, resistors, capacitors and the like which are fabricated by applying a pattern of a suitable thick film ink on the surface of a substrate and then firing the thick film ink pattern to form the thick film electrical component.
The thick film inks which are conventionally used to form thick film electrical components are comprised of a glass frit, a finely divided particulate material having certain specified electrical properties and an organic vehicle. The thick film inks which are used for each type of electrical component are specially formulated so that the final fired composition will have certain predetermined electrical properties. For example, the thick film inks used to form conductor lines are typically comprised of from about 65 to 90% by weight of a highly conductive, finely divided, powdered material, such as powdered copper or gold, a fuseable glass frit and an organic vehicle. The crossover inks have a formulation which is similar to that of the conductor inks with the exception that they typically contain in place of the conductive metal powder an insulative oxide such as aluminum oxide or the like. The resistor inks are also similar to the conductor inks with the exception that they typically contain in place of the conductive metal powders various amounts of high resistivity oxide particles.
One of the major problems encountered in the manufacture of thick film electrical components is concerned with the removal of the organic vehicle prior to firing the glass frit. If the organic vehicle is not completely removed prior to firing, organic materials will be trapped in the fused glass frit and may decompose into carbonaceous materials. This can cause undesired changes in the electrical properties, such as nonuniform electrical conductivity, shorts and opens, as well as a generally overall lower density of the glass. Furthermore, when the substrate is refired, such as to form additional thick film electrical devices on the substrate, the organic residues remaining in the fired thick film electrical components may decompose into gases which then form bubbles, voids, or even holes completely through the fired thick film electrical component. These can likewise cause shorts or high resistance areas in the thick film electrical components.
The problem of removing all of the organic materials from the patterned glass layer prior to firing is especially difficult when the starting thick film composition contains readily oxidizable materials, such as particles of metallic copper or silver, in addition to the organic vehicle. The organics can be readily removed by heating at elevated temperatures of, for example, 400.degree. to 500.degree. C. in an oxidizing atmosphere. However, if a thick film composition containing highly oxidizable materials, such as copper as noted above, in addition to the organics is heated at an elevated temperature in an oxidizing atmosphere, the readily oxidizable materials will be simultaneously oxidized. The undesirable oxidation of certain of the materials in the thick film ink such as copper makes it extremely difficult to maintain the quality of the final product. The electrical conductivity of metals such as copper is substantially reduced if they are oxidized and it becomes extremely difficult to sinter the metal particles during the final firing step. The resulting oxides on the metal particles of the thick film components also substantially reduce the solderability of the fired thick film components.
In order to reduce the effects caused by firing in an oxidizing atmosphere it has been suggested to fire the thick film inks in non-oxidizing atmospheres, such as a nitrogen atmosphere, to limit the oxidation of the other materials in the ink. The use of a non-oxidizing atmosphere to remove the organic materials is both significantly more expensive and it is difficult to prevent at least some oxidation of the material due to the minor amounts of oxygen which are inherently present in the process. Furthermore, using a non-oxidizing atmosphere in which to heat the thick film inks to remove the organics is not completely satisfactory in that considerable amounts of the organics are thermally decomposed to carbonaceous materials which then remain in the final product causing shorts and the like as noted above.
It would be highly desirable if a method could be provided for effectively removing the organic vehicle from thick film inks after application and prior to firing so as to reduce or eliminate undesirable incorporation of organic materials in the final product and to improve the uniformity and density of the thick film electrical components which are formed.